The present invention relates to a charged particle beam apparatus which measures micro-dimensions, such as circuit patterns of a semiconductor apparatus. More particularly, the present invention is concerned with a method for capturing images through measuring and adjusting an electron beam landing angle with respect to a sample and also concerned with the charged particle beam apparatus.
A CD-SEM (critical dimension scanning electron microscope) designed specifically for semiconductor is used for pattern dimension management in a semiconductor manufacturing process. The principle of the CD-SEM is shown in FIG. 2. A primary electron beam emitted from an electron gun is thinly converged by a convergence lens, and the thus converged electron beam two-dimensionally scans a sample with a scanning coil. An electron beam image can be obtained by capturing secondary electrons generated from the sample by electron beam irradiation, by use of a detector. Since secondary electrons are largely generated at edges of the pattern, portions corresponding to edges of the pattern in the electron beam image become bright. The magnification ratio of a scanned image can be changed with the ratio of the scanning width (constant) on the CRT to the scanning width (variable) of the electron beam on the sample. In the CD-SEM, a position where dimensions are to be measured with the scanned image is specified, and dimensions are measured through calculation based on the magnification ratio by use of a signal waveform obtained from the position.
Diverse methods of automatic dimensional measurement using a signal waveform have been proposed. As a typical method, a “threshold method” is shown in FIG. 3. With a pattern having a trapezoid-shaped cross section as shown in FIG. 3, the amount of signal for a portion corresponding to the side wall of the pattern forms a large signal waveform. Each portion having a large amount of signal on the left-hand side and on the right-hand side is referred to as a left white band (left WB) and a right white band (right WB), respectively. With the threshold method, maximum and minimum values are obtained in each of the right and left WBs. In addition, a threshold is calculated from these above values, and a position at which a signal waveform crosses the threshold is detected as an edge position. The distance between the right and left edges is recognized as dimensions (CD value).
A process of general automatic dimensional measurement is shown in FIG. 4. A wafer is placed on a stage (Step 101). The stage is moved to a position near the dimensional measurement section (Step 102), and images are acquired with a low magnification ratio of about 10000× (Step 103). The precise position of the dimensional measurement section is obtained through pattern recognition using registered images as templates (Step 104). Through limiting the scanning range of the primary electron beam to a narrower range centering on the obtained position (Step 105), images are acquired with a high magnification ratio of about 150000× (Step 106), and dimensions are measured (Step 108). The above-mentioned operation for changing the imaging position by changing the scanning position of the primary electron beam without moving the stage is referred to as an “image shift”. High-power images are acquired through the image shift after acquiring low-power images instead of acquiring high-power images at first since it is difficult to include a pattern to be measured in high-power images generally due to insufficient stop accuracy of the stage. The increasing of fineness of semiconductor patterns has increased the demand for improved accuracy in measurement of the CD-SEM. There have been a demand for measurement reproducibility of a single apparatus, and a plurality of apparatuses are used in many cases due to the relationship between the throughput and semiconductor production volume. For this reason, it is important to reduce the difference in dimensions measured by apparatuses (instrumental error).
In relation to the SEM, correction of aberration of the primary electron beam is presented in Japanese Patent Laid-open No. 2004-127930 and a method for obtaining an electron beam landing angle through calculation is presented in, for example, “Development of Beam Tilt Angle Calibration Technology in a CD-SEM” on pages 48 to 53 in the collected papers of View2004 vision technology application workshop lecture.